Some electronic devices have non-volatile memory such as EEPROM in which important data is stored. For example, in a mobile phone, its non-volatile memory stores information unique to that mobile phone, such as its telephone number and serial number (manufacturer's number), and management information such as a password for identifying the owner of that mobile phone to restrict the use of the mobile phone to its user.
Recently, mobile phones providing various services such as Internet connection services have become popular. In such a mobile phone, a user registration number, which is management information required for using such services, is stored in non-volatile memory. Service providers can use the user registration number to identify the user and manage information about usage-based service charges.
In a personal computer (hereinafter called “PC”), management information such as a user ID for identifying its owner and a serial number unique to the PC is stored in its non-volatile memory. These user ID and serial number are used for security control to prevent unauthorized use of the PC by another person.
Some non-volatile memories contained in electronic devices may exhibit unstable memory retention due to a defect and data such as management information stored may not properly retained. Such failures may occur particularly when using EEPROM. EEPROM has the correct content kept therein just after management information has been written. However, a read error can occur, for example, when the user attempts to read out management information several days after it is stored by a write operation because it has not been held properly.
The various items of information described above are typically important information and if it contains any errors, various kinds of problem may arise. For example, if an error occurs in the telephone number of a mobile phone, telephone calls made through the phone can be charged to another user. If an error occurs in the user ID of a PC, the ID may be considered not matching the registered one even though the owner of the PC correctly inputs the ID that he or she has registered.
It is required that management information read from electronic devices be reliable. However, prior-art memory systems cannot reliably detect an error in read management information and the read management information therefore is not always reliable.